Launched using a new booster with the same heavy lift capacity (45 metric tons) as the Saturn V rockets, the earth return vehicle is sent on a six month journey to mars.

The ERV consists of a small nuclear reactor mounted on a robotic light truck, an automated chemical processing unit with a set of compressors, a few scientific rovers, some fuel tanks, and the crew cabin for return to earth.

After landing at a pre-selected location, the robotic truck is used to move the nuclear reactor a few hundred meters away from the landing site, paying out cable as it goes.
The reactor is winched off the truck, and placed into a depression. Then the reactor is turned on, supplying 100 kilowatts of electricity to the chemical plant.

Now the chemical plant does it's work, using a simple Methanation process
to convert the 6 metric tons of hydrogen it arrived with into 108 metric tons of methane and oxygen.
Enough to bring the crew home, and supply an extra 12 metric tons of fuel for use in combustion powered vehicles.

Thirteen months later, the ERV is sitting fully fuelled on the surface of mars. At this point, mission control on earth deploys the rovers to find the best local landing site.
A radar transponder is then deployed to this site for the habitation module to home in on.

After the crew have spent a year and a half exploring the planet, they climb into the earth return vehicle, and launch themselves on a six month journey back to earth.